Radar and sonar systems identify targets and the range of targets by transmitting energy toward the target, and measuring the time between the transmission and reception of an echo from the target. Since the transmitted energy tends to spread out as it leaves the transmitter, the power density of the transmitted energy decreases with increasing distance from the transmitter. The transmitted power density is much attenuated when it reaches a target at a great distance from the transmitter. A portion of the attenuated power impinging upon the target is reflected toward a receiver, ordinarily located at the transmitting site, and the power density is again attenuated as it expands through space. When the power arrives at the receiver, it has a very small amplitude, and detection of the signal representing the target in the presence of unavoidable noise and clutter signal components remains one of the major problems in radar system design. Clutter refers to echoes from relatively large, slowly-moving targets, often close to the transmitter site, such as trees moving in the wind, or, in a marine context, from waves.
One way to increase the received signal magnitude to aid in target detection is to increase the transmitted signal magnitude. A possible way to accomplish this increase is to accumulate the energy available for transmission over a predetermined time interval, and to transmit all of the accumulated energy in the form of periodic pulses of relatively large magnitude. Once the magnitude of the transmitted pulses reaches a certain level, it may be more economical to lengthen the pulse duration rather than maintain the same duration and further increase its magnitude. Increasing the pulse duration, however, may tend to reduce the range resolution, which is the accuracy with which the range can be determined. A technique involving frequency dispersion, as by transmitting a variable-frequency "chirp" pulse, allows use of pulse-compression filters at the receiver to reduce the effective pulse duration, to thereby restore range resolution.
Range resolution is also degraded by the sidelobe structure inherent in the sharp-edged pulses produced by pulse techniques. These range sidelobes cause echo signals originating from a target associated with one range "bin" to "leak" into other range bins, such as the adjacent range bins. When one target is large, and produces a large echo signal, the range sidelobes associated with the compressed pulse associated from the echo from the large target may undesirably obscure the corresponding pulse representative of an echo from a small target in an adjacent range bin. In the case of a number of adjacent range pulses which contain echoes from high reflectivity phenomena, these traces will interfere with the measurement of the spectrum of the weather phenomenon in a resolvable range bin (or in an azimuth or elevation volume) which is somewhat removed from them. The interference in the resolvable range, azimuth or elevation volume arises from "clutter flooding", due to the presence of pulse compression sidelobes in the resolvable range bin being measured. One way to reduce the effects of range sidelobes is to reduce the magnitude of the range sidelobes themselves, which can be accomplished by applying a weighting function to a series of the pulses which are pulse compressed. Another technique of range sidelobe suppression which has been used to tend to reduce the effects of masking of targets by large adjacent targets is to apply coding to the transmitted pulse, so that the coding appears in the received echo pulse, and to apply code-matched filtering to the compressed received pulses.
Among the problems in radar-type signal processing is that functions other than detection and ranging are ordinarily performed. For example, Doppler filtering is often performed, to aid in identifying moving targets by suppressing clutter, and to distinguish among targets moving with different radial velocities. These additional processing steps, in turn, give rise to issues relating to the ordering of the processing. A system is described in U.S. Pat. No. 5,173,706, issued Dec. 22, 1992 in the name of Urkowitz, incorporated herein by reference, in which pulse compression precedes filtering by a bank of Doppler filters, and in which the Doppler-filtered signals are either at baseband, or are converted to baseband by multiplication by an exponential or oscillatory signal, in order to reduce the Doppler frequency shift across the filter bandwidth, and in order to reduce cost by permitting all of the filters of the Doppler filter bank to be identical baseband filters. The converted signals are then applied to range sidelobe suppressors. One of the problems with range sidelobe suppression techniques is that they tend to be sensitive to Doppler shifts in the echo pulse.
Another method for reducing range sidelobes is described in U.S. Pat. No. 5,151,702, issued Sep. 29, 1992 in the name of Urkowitz, herein incorporated by reference, in which the transmitted pulses are organized into mutually complementary sets, and in which the mutually complementary sets of pulses are sequentially Doppler filtered, and the filtered pulse sets are, in turn, compressed by filtering matched to the coding. After matched filtering, the resulting mutually complementary compressed pulses are summed, with the result that the main range lobes add, and the undesired range sidelobes cancel. Copending U.S. Pat. No. 5,376,937, filed Jun. 21, 1993 in the name of Urkowitz, and entitled DUAL-FREQUENCY, COMPLEMENTARY-SEQUENCE PULSE RADAR, describes a radar system in which transmission takes place simultaneously at two different frequencies, and in which each of the transmissions is coded with one of mutually complementary codes. This arrangement includes reduced processing time among its advantages.
Doppler filtering may be performed by discrete, inductance-capacitance filters, but modern systems generally use digital signal processing. U.S. Pat. No. 5,343,208, filed Dec. 22, 1992 in the name of Chesley, and entitled RADAR WITH INDIVIDUALLY OPTIMIZED DOPPLER FILTERS, describes a system in which an FFT-like array structure includes weighting elements, and in which the weights are established by a technique in which the input signals are assigned, and in which the output signals are correspondingly assigned in a fashion which defines the desired filter shaping. The actual output signals in response to the assigned input signals are subtracted from the desired output signals to form an error signal set, and the error signal set is recurrently back-propagated through the array to set the weights.
In some situations, the largest amount of clutter occurs at Doppler frequencies at which maximum range sidelobe suppression does not occur. In other words, if the clutter motion is such that the echoes occur at frequencies at which the integrated sidelobe levels are not a null, the clutter signals from one range bin will contribute to the total signal output in adjacent range bins. The presence of clutter, as mentioned above, tends to obscure point targets such as aircraft and missiles, and weak targets such as meteorological phenomena, in the presence of range extended interference such as chaff, precipitation, and sea and ground clutter echoes. This concealment will be especially significant for point targets moving nearly tangentially to the radar system, since their Doppler frequencies will be small, and will lie close to the frequencies of weather phenomena such as storms. Similarly, the presence of weather-phenomena clutter may obscure weak point targets such as aircraft and missiles.